The present invention relates to a wide belt grinding machine for boards, especially for wood panels, with at least one endless abrasive belt arranged above and/or below a plane in which the boards are moved. A machine of this general type is shown, for example, in Austrian Pat. No. 244,794, wherein each abrasive belt is driven by a motor and is tensioned by a tension roll that is connected with the piston of a piston-cylinder arrangement. The arrangement is under dual control by compressed air with the position of the cylinder piston being adjusted by means of a conventional control element such as a valve or the like.
Although wide belt grinding machines of the aforementioned type have proven themselves in commercial use, difficulties have been encountered from time to time as in the case, for example, of power failures or other electrical and mechanical disturbances which can lead to machine damage and abrasive belt damage. In such situations, serious trouble has been encountered if an abrasive belt comes into contact with any metallic parts because of spark production which gives rise to the possibility of a fire, especially since there is an ample spray of sparks in such instances. Occasionally, the so-called oscillatory control for constantly moving the abrasive belt to and fro at right angles to its travel direction fails to function properly and is usually also attributable to some electrical malfunction.
Any type of power disturbance which interrupts the operation of such a machine is inconvenient, particularly an electrical power failure. Even though the motors no longer receive any electrical current in the case of a power failure, the driven parts still continue to operate for a period of time. Thus, some provision has had to be made for including a pneumatically or hydraulically operated motor brake that would not be affected by an electrical power failure. Such motor brakes are, on the one hand, relatively expensive and themselves prone to disturbances, and, on the other hand, the time required for the complete arresting of the abrasive belts is too long due to the large rotating masses to prevent with certainty an untrue running of the tensioned abrasive belts. Also, such wide belt grinding machines have already been connected with emergency power supply units serving at least to maintain the oscillatory control of the abrasive belt during power failures. However, these units are likewise relatively expensive and require constant servicing to keep them in operation. Therefore, the problem of a lateral untrue running of the abrasive belts in case of malfunctions has not as yet been solved satisfactorily.